Development of the modern pH scale was first discussed in a 1909 paper by a Danish scientist named Soren Sorenson. Sorenson proposed that the actual degree of acidity should be rationally measured by hydrogen ion concentration and created the pH scale for expressing these levels. Today, it is measured on a scale of 0 to 14 with the lower numbers being more acidic, the higher numbers more basic, and 7 as the neutral value. In chemical terms, pH means the negative log of the concentration of protons in solution.
A commonly used tool for identifying pH of liquids is the use of pH papers or indicators. These papers, when exposed to a liquid media, will change color as the pH level varies. These indicators are convenient to use, but have limitations on their accuracy, and can be difficult to interpret correctly when used with a colored or murky sample.
To obtain more accurate readings, one typically relies on electronic pH measurement equipment. This equipment consists of three parts: a pH measuring electrode, a reference electrode, and a high input impedance meter. The pH electrode can be thought of as a battery, with a voltage that varies with the pH of the measured solution. Commonly, the pH measuring electrode is a relatively large glass bulb with a hydrogen ion sensitive coating. This coating will create a millivolt output that varies with changes in relative hydrogen ion concentration inside and outside of the bulb. The reference electrode can consist of a combination of metals and chemicals that create a millivolt output that does not vary with changes in hydrogen ion concentration.
In addition to coated glass, there exists many other types of pH sensing electrodes. Metallic substances such as antimony, that exhibit a change in electrical potential when immersed in different pH fluids, can be used. Other materials such as specially formulated polymers have also been used successfully.
Semiconductor technology can be used to create transistors that can sense pH changes in fluid. Ion Sensitive Field Effect Transistors (“ISFET's) typically exhibit improved repeatability and precision over a wide dynamic range, though at a considerably higher cost.
Other state of the art devices utilize optical sensing, capacitive sensing, and nanotechnology.